Lewis diagrams | The periodic table and Lewis diagrams | High school chemistry | Khan Academy

- [Instructor] In this video,

we're gonna introduce
ourselves to a new way

of visualizing atoms.

And as you can imagine
from the title here,

that's going to be Lewis diagrams.

But before I even get into that,

let's do a little bit of
review of what we already know

about Bohr models.

So let's say we take an
arbitrary element here.

Let's say we take nitrogen.

Nitrogen, by definition,
has seven protons.

And so if it's neutral, it's
going to have seven electrons.

So a Bohr model for nitrogen,

in our first shell, that
first shell is going to look

just like helium and it's
going to have two electrons.

So let me draw it like that.

And then in its second shell,

its second shell, it is going
to have the remaining five

of the seven electrons.

And we are going to make
them unpaired at first.

So one, two,

three, four, and then five.

The reason why I did it this way is,

a full valence shell is
going to have eight electrons

or four pairs.

But if the electrons can spread apart,

they like to spread apart.

So that's why I did one, two, three, four,

and then I paired this last one
because there's nowhere else

for it to actually go.

Now, I just touched on this
issue of valence electrons.

Those are the electrons
in your outermost shell,

and they tend to be the ones
that are involved in reactions.

So chemists said, "Hey,
just for shorthand,

instead of having to draw
all of this every time,

why don't we just visualize
the valence electrons?"

And so let's do that in
this nitrogen example.

So a Lewis diagram,

which is I'm just going to draw right now,

is that simplified visualization

where you write the
symbol for that element,

and you just depict its valence electrons.

We just saw that there
are five valence electrons

for nitrogen, seven
total, but five valence,

five electrons in that outermost shell.

So it is going to be one, two,

three, four, and then five.

So that's a Lewis diagram
for a neutral nitrogen atom.

It turns out we can also do this for ions.

So let's say that we had
a nitride ion over here.

Now, a nitride ion has
gained three electrons.

So it actually has
eight valence electrons.

So if you gain three from five,
you're going to have eight.

So I'll go one, two,

three, four, five,

six, seven, eight.

And because it gained three
electrons from being neutral,

it now has a negative three charge.

And so you'll often see
it written like this

where they put brackets around it,

and you would see three minus.

Now, the last thing that
you might wonder about is,

"Okay, I kinda understood

how you got the valence
electrons for nitrogen.

Is there just some general
pattern in the periodic table?"

And the simple answer is yes.

And that's one of the useful things

about the periodic table.

Or as we'll learn,
there's many, many other

really interesting things about it.

If you look at the groups, in general,

you're going to have one valence electron

for Group One elements,
for this column over here.

You're going to have two valence electrons

for these Group Two elements.

And I know what you're thinking,

"Okay, is just the group the
number of valence electrons?"

Well, unfortunately, it doesn't
exactly work out that way.

I'm going to skip the
transition metals here

because those get a little
bit more complicated.

It's a little bit more advanced.

But then if we go over
here to, what is this,

Group One, Two, Three,
Four, Five, Six, Seven,

Eight, Nine, 10, 11, 12, 13,

Group 13 over here is going to
have three valence electrons.

Group 14, four valence electrons.

Five valence electrons in Group 15,

and that's why we saw
five valence electrons

for nitrogen here.

Six for Group 16.

Seven for Group 17.

And then 18 for, or sorry, (laughing)

I should say eight valence
electrons for Group 18.

So one way to remember it is,

for Groups 13 through 18,

you take the group number
and you subtract 10,

and you're going to get the
number of valence electrons.

And hopefully that made sense based

on how we were able to figure
out the valence electrons

for example nitrogen.